1. Field of the Invention
The present invention relates to a solid-state image-sensing device that is switchable between a first state in which it outputs an electric signal that is linearly proportional to the amount of incident light and a second state in which it outputs an electric signal that is natural-logarithmically proportional to the amount of incident light.
2. Description of the Prior Art
Conventional solid-state image-sensing devices are grouped roughly into CCD-type and MOS-type according to the means by which they read out photoelectric charge generated in photoelectric conversion elements. CCD-type image-sensing devices transfer electric charge while accumulating it in potential wells. On the other hand, MOS-type image-sensing devices read out electric charge accumulated in the pn junction capacitance of photodiodes through MOS transistors. However, these conventional solid-state image-sensing devices yield outputs that are proportional to the amount of electric charge generated, and thus have the disadvantage of narrow dynamic ranges.
To obtain a wider dynamic range, a solid-state image-sensing device has been proposed that is provided with a photosensitive means that produces a photocurrent commensurate with the amount of incident light, a MOS transistor to which the photocurrent is fed, and a bias means that biases the MOS transistor in such a way that a subthreshold current flows therethrough. This solid-state image-sensing device outputs an electric signal that is natural-logarithmically proportional to the amount of incident light. This type of solid-state image-sensing device has a wide dynamic range, but has the disadvantages of unsatisfactory characteristics in low-brightness condition and an insufficient SIN (signal-to-noise) ratio.
On the other hand, an optical sensor circuit has also been proposed that is provided with a photosensitive means that produces a photocurrent commensurate with the amount of incident light and a MOS transistor to which the photocurrent is fed; in addition, this optical sensor circuit is so configured as to be switchable between a first state in which it yields an output linearly proportional to the photocurrent and a second state in which it yields an output natural-logarithmically proportional to the photocurrent (U.S. Pat. No. 5,861,621).
In this optical sensor circuit, proposed in U.S. Pat. No. 5,861,621, that can be switched between linear conversion and logarithmic conversion, a node between a photodiode and a capacitor is reset by making the gate voltage of a MOS transistor sufficiently higher than the drain voltage thereof so that the drain-source channel of the MOS transistor is brought into a low-impedance state. This makes the potential at the source substantially equal to the potential at the drain. As a result, where a plurality of such circuits are provided, the node between the photodiode and the capacitor is reset in such a way that the voltage there becomes identical in all the circuits. That is, the variations in sensitivity among the individual circuits due to the variations in the threshold voltage of their MOS transistors are not reflected in the outputs from those circuits. This causes the switching point between linear conversion and logarithmic conversion to vary from one pixel to another.